Method for automatic switching of a communication resistor of a hart device in or out

ABSTRACT

Method for automatic switching of a communication resistor of a HART device in or out, wherein the method comprises steps as follows: transmitting a test signal on an electrical current loop; reading from the electrical current loop a test voltage signal, which is based on the transmitted test signal; comparing the test voltage signal with a reference signal; connecting the communication resistor into the electrical current loop when the test voltage signal exceeds the reference signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present continuation application is related to and claims thepriority benefit of U.S. patent application Ser. No. 15/567,261, filedon Oct. 17, 2017, International Patent Application No.PCT/EP2016/056561, filed Mar. 24, 2016, and German Patent ApplicationNo. 10 2015 105 887.1, filed on Apr. 17, 2015, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a method for automatic switching of acommunication resistor of a HART device (for example, a HARTcommunication box) in or out.

BACKGROUND

In process technology as well as in automation technology, field devicesare often applied, which serve for registering and/or influencingprocess variables. Serving for registering process variables aremeasuring devices, such as, for example, fill level measuring devices,flow measuring devices, pressure- and temperature measuring devices, pHmeasuring devices, conductivity measuring devices, etc., which registerthe corresponding process variables, fill level, flow, pressure,temperature, pH-value, and conductivity, respectively. Used forinfluencing process variables are actuators, such as valves or pumps,via which e.g. the flow of a liquid in a pipeline or the fill level of amedium in a container is changed. Referred to as field devices are, inprinciple, all devices, which are applied near to the process and whichdeliver, or process, process relevant information. A large number ofsuch field devices are available from the group of firms,Endress+Hauser. In connection with the invention, the concept, fielddevice, thus, refers to all types of measuring devices and actuators.Furthermore, the concept, field device, includes, however, also e.g. agateway, a radio adapter or other bus participants integrated/integrablein a bus system.

Control occurs, in such case, from a control system, which is connectedwith the individual field devices via a data bus. All information neededfor the control are exchanged between the control system and the fielddevices via the data bus, also called a fieldbus.

An often applied data-, or fieldbus, works according to the HARTstandard of the HART® Foundation. Field devices working according to theHART standard are also referred to as HART devices.

Besides the transmission of measured values, field devices allow alsothe transmission of other kinds of information stored in the fielddevice, information such as e.g. configuration- and/or parameter data(zero-point, measured value range, etc.), measurement curves as well asdiagnostic information. The parameters can be set by the control systemor from a service- and display device.

This procedure is also referred to as configuring and/or parametering ofthe field device. The data for this are referred to as configuration-and/or parametering data.

In the case of each initial installation or in the case of a devicereplacement, the field device must be configured and/or parametered.

For this, a service- and/or observation software runs, most often, on aportable computer unit (PC, laptop), which is connected, for example,via a USB- or also a serial, COM interface (e.g. RS232, RS485) with anapparatus connected to the fieldbus, a so-called HART communication box,for transmission of the configuration- and/or parametering data.

In order to establish a HART-standard conforming communication betweensuch a HART communication box and a field device, an impedance matchingis required, since only such a so-called “low-impedance” (acronym: LI)installation is permitted within the corresponding electrical currentloop.

Since current field devices have both “low-impedance” as well as also“high-impedance” (acronym HI) interfaces, a service technician must takecare that the HART communication box is appropriately matched andconnected to the electrical current loop. This can occur, for example,using a particular HART communication box, in the case of which acorresponding communication resistor (also called the load) isswitchable in by means of a switch. Alternatively, a service techniciancan manually introduce a corresponding resistor into the existinginstallation.

Both variants assume that the service technician knows whether acommunication resistor must be connected or whether a communicationresistor is already present and, thus, does not have to be switched in.

In the case of both variants, for example, by incorrect positioning ofthe manual switch of the HART communication box or also by manualintroduction of an additional communication resistor not actuallyrequired, problems can arise as follows:

1.) If the installation, for instance in the electrical current loop,two communication resistors are connected in parallel when they shouldnot be, the impedance and therewith the signals, i.e. the signalamplitudes, can lessen in such a manner that communication is no longerpossible.

2.) If in the installation no communication resistor is present and two“high-impedance” devices are connected together, the signals, i.e. thesignal amplitudes, get larger and the inputs of the communication boxcan be overloaded. This can lead to communication problems or even tocomplete loss of communication.

An object of the invention is to enable an as defect free as possibleHART communication connection.

SUMMARY

The object is achieved by a method and a HART device.

As regards the method, the object is achieved by a method for automaticswitching of a communication resistor of a HART device in or out,wherein the method has steps as follows:

-   -   transmitting a test signal on an electrical current loop;    -   reading from the electrical current loop a test voltage signal,        which is based on the transmitted test signal;    -   comparing the test voltage signal with a reference signal;    -   connecting the communication resistor into the electrical        current loop when the test voltage signal exceeds the reference        signal.

According to the invention, thus the communication resistor of the HARTdevice, for example, a HART communication box, is switched in or out.The automatic switching in or out of the communication resistor, i.e.the load, assures an as defect free as possible communication connectionand prevents the above described problems. Thus, the application of theHART device is simplified and its operability improved.

An advantageous form of embodiment of the method of the inventionprovides that a preamble within a HART communication is used as testsignal.

The HART protocol is based on transmission of data telegrams, which areintroduced by a so-called preamble phase, in which at least five to atmost 20 characters with the value 0xFF are sent. The preamble serves, insuch case, for synchronization between master and slave.

An advantageous form of embodiment of the method of the inventionprovides that the read test voltage signal is filtered, before it iscompared with the reference signal. In such case, a first variantprovides that the test voltage signal is filtered through a bandpassfilter with a center frequency of about 1200 Hz and a second,alternative variant provides that the test voltage signal is filteredthrough a lowpass filter with a limit frequency of about 1200 Hz.

An advantageous form of embodiment of the method of the inventionprovides that the comparing of the test voltage signal with thereference signal occurs over a predefined duration and only when thetest voltage signal exceeds the reference signal for the predefinedduration is the communication resistor connected.

Another advantageous form of embodiment of the method of the inventionprovides that before the transmitting of a test signal, thecommunication resistor is switched out.

Another advantageous form of embodiment of the method of the inventionprovides that, after switching in of the communication resistor, acarrier detect signal is used for verification and, when the carrierdetect signal indicates a valid carrier signal, the communicationresistor remains switched in. Especially, it can be provided that whenthe carrier detect signal indicates an invalid carrier signal, thecommunication resistor is switched out.

As regards the HART device, the object is achieved by a HART device witha communication electronics, which is designed for performing the abovedescribed method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail based on theappended drawing, the figures of which show as follows:

FIG. 1 shows a schematic representation of a HART communication boxknown from the state of the art and connected to an electrical currentloop, or a control loop, with a communication resistor,

FIG. 2 shows a schematic representation of the HART communication box,which is connected via a communication resistor to the electricalcurrent loop, or the control loop, and

FIG. 3 shows a schematic representation a HART device of the inventionin the form of a HART communication box with a communicationelectronics, which is designed to execute the method of the invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show the two connection variants known from the state ofthe art of a HART communication box 1 connected to an electrical currentloop, or to a control loop, 3. In such case, the HART communication box1 has been selected for purposes of explanation. The invention, is,however, in principle, applicable for any HART device.

FIGS. 1 and 2 show further, schematically, an installation in principle,in the case of which at least one field device 4 is connected via anelectrical current loop 3 with at least one superordinated unit, forexample, a control system or, such as shown in FIG. 1, a service unit 2.

In FIG. 1, the communication resistor 6 required for conforming HARTcommunication is located within the HART communication box and notseparately shown.

In contrast therewith, FIG. 2 shows an installation, in the case ofwhich the required communication resistor 6, for example, one manuallyintroduced by a service technician, is already present in the electricalcurrent loop 3. In this case, the voltage signal is tapped across thisresistor by the HART communication box 1.

Both the HART communication box 1 shown in FIG. 1 as well as also thatshown in FIG. 2 have a first interface 15, which is embodied, forexample, according to the USB-standard, and a second interface 16 in theform of connection terminals.

The first interface 15 connects the HART communication box 1 with theservice unit 2, which serves for configuration and/or parametering of afield device 4. The USB connection shown in FIG. 1 is only by way ofexample, and other forms of communication connection, e.g. RS232,Bluetooth, etc. likewise provide options.

The HART communication box 1 is connected to the electrical current loop3 via the second interface 16. Typically for this, the second interface16 is implemented in the form of connection terminals.

Via the two interfaces 15 and 16, the HART communication box 1 connectsthe portable computer unit, e.g. service unit 2, with the electricalcurrent loop 3 data conductively, so that via the service unit 2, onwhich a correspondingly provided service- and/or observation software isrunning, and the electrical current loop 3, it is possible tocommunicate with a field device 4 connected to the electrical currentloop 3.

FIG. 3 shows a schematic representation of a HART device of theinvention, i.e. a HART communication box 1 with a communicationelectronics located within the communication box 1. The communicationelectronics includes a computing unit, for example, a microprocessor 13and a HART modem 9, which together implement the transfer of the data,especially configuration data and/or parametering data, between thefirst interface 15 and the second interface 16. In this way,configuration data and/or parametering data is transferred between thefirst interface 15, which is embodied according to the USB standard, andthe second interface 16, which is embodied according to the HARTstandard.

In order to be able to practice the method of the invention describedbelow, the communication electronics further includes a communicationresistor, which can be automatically connected in or out via a switchoperated by the microprocessor, and a comparator. Additionally, thecommunication electronics includes a filter element, for example, abandpass filter- or lowpass filter. The filter element serves to filterout disturbance signals, which are produced, for example, by externalEMC influences and, thus, to prevent an unintended influencing of thesignal of interest. In principle, the invention can, however, also bepracticed without the filter element.

The method of the invention assumes that one starts with the HARTcommunication box 1 in the “high-impedance” state, i.e. thecommunication resistor 6 is normally switched out. In this way, only anerroneous scenario must be recognized: the defective connection of twoHI devices, thus, for example, a HI field device and the HARTcommunication box without a communication resistor switched in, i.e.with communication resistor switched out. The connection of two HIdevices has the result that the signal level of the HART signal is toohigh. This is recognizable by analysis of peak values. In order toperform this analysis, a test signal is transmitted from the HARTcommunication box 1 via the connection terminals and then a test voltagesignal, which is based on the transmitted test signal, is read back.Opportunely, the preamble is used, which is composed of three or moretransmitted 0xFF hexadecimals, which serve, in this case, both forsynchronization as well as also test signal.

In order that an unintended, or erroneous, switching in of thecommunication resistor 6 is prevented as much as possible, it isadvantageous to filter the test voltage signal (typically 1200 Hzsignals) coming from the connection terminals of the HART communicationbox 1. This filtered signal is then fed to the comparator 12.

The comparator 12 serves for comparing the test voltage signal with areference signal, or with a reference level. In the case, in which thereference signal, e.g. the reference level, is exceeded, themicroprocessor connected to the comparator can use this as switchingsignal for the switching in of the communication resistor via theswitch. Typically, the switching in of the communication resistor by themicroprocessor is only performed after the reference level has beenexceeded for a predefined duration. In this way, disturbing influences(EMC, etc.) can be minimized.

In order to enable a verification of the performed measure,advantageously a carrier detect signal is used, which is output from theHART modem 9. The carrier detect signal is a control signal, which isproduced according to standard by the HART modem 9 and displays that itcan receive data. Typically, this signal can only assume the logicalstates “high” or “low”, respectively a binary one or a binary zero.

In the case, in which a valid carrier detect signal is present, it canbe assumed therefrom that the signal received via the connectionterminals is free of error. Thus, when the carrier detect signal assumesa logical binary one, or “high”, the measure, thus the switching in ofthe communication resistor 6, is deemed successful. In contrast is thecase, in which the carrier detect signal assumes a logical binary zero,whereupon the measure is deemed not successful and the switched incommunication resistor 6 is switched back out.

LIST OF REFERENCE CHARACTERS

-   1 HART device, or HART communication box-   2 service unit-   3 electrical current loop, or control loop-   4 field device-   5 measurement transmitter, power supply-   6 communication resistor, or load-   7 connection terminals-   8 switch-   9 HART modem-   10 filter-   11 reference signal-   12 comparator-   13 computing unit, microcontroller-   14 carrier detect signal-   15 first interface-   16 second interface

1. A method for an automatic switching of a communication resistor of aHART device in or out, comprising: providing a HART device, including: afirst interface configured to connect with a service unit; a secondinterface configured to connect with an electrical current loop; amicroprocessor; a HART modem having a carrier detect signal, wherein theHART modem is connected to the microprocessor, including the carrierdetect signal connected with the microprocessor; a communicationresistor; a switch controlled by the microprocessor, wherein the switchis configured in a first state to connect the communication resistorwith the second interface, and wherein the switch is configured in asecond state to disconnect the communication resistor from the secondinterface; a filter element connected to the second interface andconfigured to filter out disturbance signals from a test voltage signalreceived via the second interface; and a comparator connected to anoutput of the filter element, wherein the comparator is configured tocompare the filtered test voltage signal with a reference signal, andwherein an output of the comparator is connected to the microprocessor,wherein the microprocessor is configured to transmit a test signal on anelectrical current loop, and to set the switch to the first state or tothe second state based on the comparison of the filtered test voltagesignal with the reference signal; connecting the HART device to anelectrical current loop; transmitting a test signal on the electricalcurrent loop; reading from the electrical current loop, via the secondinterface, a test voltage signal that is based on the transmitted testsignal; filtering, via the filter element, disturbance signals from thetest voltage signal; comparing, via the comparator, the filtered testvoltage signal with a reference signal; and connecting the communicationresistor into the electrical current loop or disconnecting thecommunication resistor from the electrical current loop based on thecomparison of the filtered test voltage signal with the referencesignal.
 2. The method as claimed in claim 1, wherein a preamble within aHART communication is used as the test signal.
 3. The method as claimedin claim 1, wherein the test voltage signal is filtered through abandpass filter with a center frequency of about 1200 Hz.
 4. The methodas claimed in claim 1, wherein the test voltage signal is filteredthrough a lowpass filter with a limit frequency of about 1200 Hz.
 5. Themethod as claimed in claim 1, wherein the comparing of the filtered testvoltage signal with the reference signal occurs over a predefinedduration and only when the filtered test voltage signal exceeds thereference signal for the predefined duration is the communicationresistor connected.
 6. The method as claimed in claim 1, furthercomprising: disconnecting the communication resistor from the electricalcurrent loop before the transmitting of a test signal.
 7. The method asclaimed in claim 1, further comprising after the connecting of thecommunication resistor: verifying a carrier detect signal indicates avalid carrier signal; and if the carrier detect signal indicates novalid carrier signal, disconnecting the communication resistor from theelectrical current loop.